A wire electrical discharge machine exists that causes an electrode wire composed of a metallic wire to face a workpiece with a minute gap therebetween and performs a cutting process on the workpiece by using discharge energy. An important part of the technology for realizing automation of wire electrical discharge machining is an automatic wire connecting device that passes the electrode wire through a machining start hole of a workpiece before machining is started.
With the use of an automatic wire connecting device, when an electrode wire is broken during machining, machining can be resumed by reconnecting the electrode wire. Moreover, an electrode wire can be automatically inserted into a different machining position of the same machining material or into a new workpiece supplied after machining is finished. Therefore, a wire electrical discharge machine can be operated unattended anytime.
In order to realize the automatic connection function of an electrode wire, an electrode wire needs to be automatically passed from the upper wire guide to the lower wire guide through a groove or a prepared hole of a workpiece. However, the electrode wire is wound on the bobbin before being connected and therefore becomes curled due to the way it is wound on the bobbin. Therefore, the tip of the electrode wire deviates from the groove or the hole and thus cannot be inserted into the prepared hole or the like in some cases.
In view of the above, the straightness of an electrode wire is improved by performing a heating process (annealing) by applying current to the electrode wire before the electrode wire is inserted. When annealing is performed, the temperature of the electrode wire increases; therefore, thermal expansion occurs due to the heating and the electrode wire is deflected. If annealing is performed on the bent electrode wire without removing the deflection due to the thermal expansion, the straightness of the electrode wire cannot be improved. In order to perform annealing while keeping the electrode wire straight, there is a technology, for example, as disclosed in Patent Literature 1, in which a heating process is performed while applying tension to the electrode wire.